1. Field of the Invention
The present invention relates to a device for driving a thermal head which is used with a thermal printer or the like and, more particularly, to a device for controlling the drive of heat-generating resistance elements, which are arranged in an array to constitute a thermal head, and capable of automatically controlling the printout density to an optimum one.
2. Discussion of Background
Generally, a thermal printer includes a thermal head which has an array of heat-generating resistance elements arranged to cover one whole line. The resistance elements are energized in a predetermined sequence responsive to a command from a controller so as to print out pictures, characters and others as dots on a heat-sensitive paper or a thermal transfer member. A predominant approach for thermal printing or thermal transfer printing is controlling the color-developing density or the color-developing area in dependence upon the duration or the peak of power which is supplied to the resitance elements. As generally accepted, the density or the area of color development in such a printing technology is a monotone incremental function which shows saturation with respect to the duration or the peak of power supplied to the resistance elements. In practice, however, the density or the area varies with the quantity of heat which is accumulated in a resistance element, resulting in an uneven density distrubution. Especially, those resistance elements which are horizontally aligned in the same line have such influence on each other that the attainable image reproducibility is signifiantly limited.
In light of this, some approaches for the drive and control of resistance elements have been proposed as enumerated below.
(1) In a drive control circuit for an array of heat-generating resistance elements adapted for thermal printing, means is provided for storing image signals associated with lines i, i-1, . . . , i--at the instant when power is applied to resistance elements associated with the respective pixels of the line i responsive to image signals so as to print out images which correspond to the image signals. Means is also provided for density-compensating the image signals of the line i depending upon the time interval between the start of printing the line i-1 and that of the line i, the time interval between the start of printing the line i-2 and that of the line i-1, . . . , the time interval between the start of printing the line i-t-1 and that of the line i-t. Means is further provided for controlling the duration or the peak of power to be supplied to the resistance elements. Such means cooperate to compensate the density of image signals, control the duration or the peak of power supplied to the respective pixels of the line i responsive to the compensated image signals, and thereby suppress the irregularity in density. For details of such a scheme, a reference may be made to Japanese Unexamined Patent Publication (Kokai) No. 58-146176.
(2) At a certain dot timing, those resistance elements of a print head which neighbor a resistance element energized two dots before and a resistance element energized one dot before are applied with drive pulses the width of which is narrower than the others, thereby setting up even density among the various dots. This approach is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 57-34986.
(3) In a thermal printer of the type having N heat-generating resistance elements, and applying drive pulses to the resistance elements in a predetermined sequence to cause them to generate heat to thereby print out images, information of the resistance elements concerning past, present and future heat generation is detected so that the number of drive pulses to be applied to a resistance element which is expected to generate heat is varied such that a substrate, which supports the resistance elements, is kept at a temperature lower than a predetermined one. This approach is directed toward preventing running of printed images due to temperature elevation of the substrate, as disclosed in Japanese Unexamined Patent Publication No. 57-117978.
The problems with all the approaches (1)-(3) discussed above is that a significantly complicated control circuit is required in order than the density may be compensated by varying the duration of power supply to resistance elements, pulse width, peak or the like in dependence upon the surrounding conditions.